Refraction and Lens

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Refraction and Lens
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Refraction

• Refraction the change in direction of a wave as
  it crosses the boundary b/w 2 media in which a
  wave travels different speeds
• the bending of waves b/c one part of each wave is
  made to travel slower (or faster) than another
  part.

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• Ex Your legs appearing shorter in a swimming
  pool a pencil appearing bent when in a glass of
  water

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• When light rays enter a medium in which their
  speed decreases (more dense medium), as when
  passing from air to water, the rays bend toward
  the normal.
• When light rays enter a medium in which their
  speed increases (less dense medium), as when
  passing from water to air, the rays bend away
  from the normal.

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Lenses

• A lens is a piece of glass or plastic that bends
  parallel rays of light so they cross and form an
  image.
• Lenses function off the principle that light
  refracts thru glass
• Light bends b/c it is changing speed going from
  one medium into another
• There are 3 types of lenses
• Plane (flat)
• converging (aka Convex)
• Diverging (aka Concave)

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Converging Lenses

• A converging lens is thicker in the middle than
  the edges.
• Convex refers to the shape of the lens
• Rays of light that are initially parallel are
  made to converge (come together)
• Converging refers to the behavior of the light
• Rays that move
• through the center
• move slower
• than rays moving
• through the edges

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Diverging Lens

• A diverging lens is thinner in the middle than
  the edges.
• Concave refers to the shape of the lens
• Rays of light that are initially parallel are
  made to diverge (spread apart)
• Diverging refers to the behavior of the light
• Rays that move
• through the center
• move faster
• than rays moving
• through the edges

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Focal Points

• The focal point is the point where light
  converges.
• In a converging lens, light passes through the
  focal point.
• In a diverging lens, light appears to go through
  the focal point. Light looks like it originates
  from a single point.

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Lens Image Formation
Lens Outside the focal point Inside the focal point
Converging (convex) Real Smaller Inverted Virtual Larger Upright
Diverging (concave) Virtual Smaller Upright Virtual Smaller Upright
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Check Your Understanding

• What type of lens is used in a magnifying glass?
• A converging lens. It will create a larger
  (magnified), upright image. Many instruments
  used for magnification, such as a microscope or
  telescope, use multiple converging lenses.

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Check Your Understanding

• Why are diverging lenses not used on overhead or
  LCD projectors?
• B/c diverging lenses only produce virtual images.
  Only real images can be projected onto a screen.

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The Eye

• The eye is similar to the camera.
• The amount of light that enters the eye is
  regulated by the iris which surrounds the opening
  called the pupil. Light enters through the
  transparent cornea through the pupil. The lens
  focuses the light on a layer of tissue on the
  back of the eye called the retina.
• The eye uses a converging lens to focus light on
  the retina.

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• In both a camera and the eye, the image that is
  formed is up side down. Our brain turns the
  image right side up for us
• Adjustments in focusing is called accommodations.
• Accommodation is accomplished by changing the
  thickness and shape of the lens.
• Our eye does this by contracting or releasing eye
  muscles

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Check Your Understanding

• Why does your brain have to flip the image
  projected onto the back of our eye?
• Because our eye uses a converging lens, which
  inverts images outside the focal point (most
  objects). Our brain has to flip the image so as
  not to confuse our balance.

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Some Defects in Vision

• The eyes of a farsighted person forms images
  behind the retina. The eyeball is too short.
• A person with farsighted vision can see objects
  that are far but not near.
• A converging lens corrects farsighted vision.

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• The eyes of a nearsighted person forms images in
  front of the retina. The eyeball is too long.
• A person with nearsighted vision can see objects
  that are near but not far.
• A diverging lens will correct nearsighted vision

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Summary of Vision Defects

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Vision Type CAN See CANNOT See Corrective Lens
Farsighted Far Near Converging
Nearsighted Near Far Diverging
Astigmatism a defect of the eye caused when the
cornea is curved more in one direction than in
another
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Total Internal Reflection

• When light travels from a more dense medium to a
  less dense medium, there is a certain angle to
  which no light will be refracted and all light
  will be reflected. This is called the critical
  angle.
• Total internal reflection is the 100 reflection
  of light that strikes the boundary between 2
  media at an angle greater than the critical
  angle.
• Ex The critical angle for glass is 43, the
  critical angle for water is 48, and the critical
  angle for a diamond is 24.6.

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• The dotted line represents the critical angle.
  The green line represents total internal
  reflection.

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• Optical fibers are transparent fibers that pipe
  light from one place to another using total
  internal reflection.
• Optical fibers are important in communications.

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Check Your Understanding

• What type of lens does one where to correct
  farsightedness?
• A converging lens because it can make objects
  appear larger (closer) than they actually are.